Here are my results for an exercise in preparing a summer weather overnight kit using the OR Dry Peak Bagger 9.20z silnylon dry-bag style pack:
I was able to get it all in the 1650ci/27 liter bag by using a 40F down bag and stowing my rolled sleeping pad (closed cell Insul Mat) on the top of the pack. My survival kit is carried on a belt bag. I was able to stash everything inside using a 48" long Thermarest Original sleeping pad folded against my back--- at a 7oz weight penalty over the Insul Mat. The Thermarest is slippery too.
I could shave a little more weight in truly hot weather by cutting back on the extra base layer and using running shorts supplemented with SUL wind pants.